This invention relates to a two-stage cryopump, including condensation and adsorption surfaces which are associated with the second (colder) stage. A two-stage cryopump of this type is known and is disclosed, for example, in German Offenlegungsschrift (application published without examination) 35 12 614.
Two-stage cryopumps are conventionally driven by a two-stage refrigerator constituting the cooling source. The first stage of the refrigerator has a temperature of 60-100K. The pumping faces (such as a radiation screen for the second stage) coupled to the first stage in a good heat conducting manner preferably serve for collecting thereon, by condensation, gases like water vapor, carbon dioxide and the like.
At the second stage of the refrigerator, which, during operation, has a temperature of approximately 10-20K, further pumping faces are provided which have directly and indirectly accessible zones. The directly accessible zone serves preferably for the removal of gases such a nitrogen, argon and the like by condensation. The indirectly accessible zone is provided for removing light gases such as hydrogen and helium by adsorption. Such an indirectly accessible pumping face zone is conventionally coated with an adsorption material, for example, active carbon.
In known cryopumps the capacity of the adsorbing pumping faces of the second stage is relatively small as compared to the capacity of the condensing pumping faces of the second stage. When such cryopumps are used in sputtering systems in which large hydrogen quantities are used, there is thus often encountered the disadvantage that the adsorption capacity is exhausted long before the condensating capacity. It is then necessary to regenerate the adsorption surfaces which is effected by energizing a heater associated with the second stage. In this manner the stage itself as well as the pumping faces are heated. A temperature increase to at least 70K, preferably 90K is necessary to achieve a complete regeneration of the adsorption surfaces. Since the condensation surfaces of the second stage too, assume such a higher temperature, it is unavoidable that condensatable gases such as argon are vaporized which thus takes place simultaneously with the regeneration of the condensation faces of the second stage.
If the regenerating process is stopped immediately after the complete regeneration of the adsorption surfaces (which only takes a short period of time) and the re-cooling of the pumping faces of the second stage is commenced, then the condensation surfaces of the second stage are not yet fully regenerated. Since the adsorption material, preferably active carbon, has still a good adsorption probability for the above-noted condensatable gases (for example, argon) at the higher temperatures in the range of 70-90K, such gases may drift at these elevated temperatures from the condensation faces onto the adsorption faces. Thus such condensatable gases occupy already in the re-cooling phase the active carbon and therefore adversely affect the adsorption capacity of the adsorbing material for light gases for which the adsorption capacity of the adsorbing material should have been reserved in the first place. In order to avoid such an adverse effect in the known cryopumps, it is necessary to carry out a time-consuming regenerating process for both pumping faces of the second stage even if only the adsorption capacity was exhausted, although such a process for the condensation surfaces would have been necessary only after a long period of time.